VTS 3D Geospatial Software Stack is a state-of-the-art, full-stack
open source platform for 3D geospatial application development. Consisting
of 3D streaming backend components and of JavaScript and C++ frontend
libraries, VTS provides for interactive 3D rendering of geospatial
content on the Web, on the desktop or on mobile. VTS is designed
and engineered at Melown Technologies.

The highlights of VTS 3D Stack are:

complete open source system fro 3D maps streaming and rendering

massively scalable and performance-oriented

server-side data intergration tools

dynamic DEM/DSM streaming

GDAL raster and OGR vectors with stylesheets

streams and renders photogrammetric 3D models (I3S, VEF, LODTree)

WMS/WMTS and Mapbox vector tile support

coordinate system agnostic with seamless polar caps

reliance on existing open source libraries (GDAL, PROJ4) and standards

Melown Cloud is a cloud 3D map development platform operated by Melown Tech atop of VTS 3D Stack. It is a point-and-click interface to a subset of VTS functionality ideal for smaller projects and less technically savvy users.

It is also a great source of custom mapconfigs for client application development.